As is well known, polyimides are plastics which contain in their main chain the grouping EQU --CO--NR--CO--
as a linear unit, or the grouping ##STR1## as a heterocyclic unit. They are condensation products of polyfunctional carboxylic acid anhydrides and primary diamines.
Typical examples of such polyimides are the polypyromellitimides, commercially available under the trade name "Kapton" (registered Trade Mark of DuPont), which can be prepared by reacting pyromellitic dianhydride with aromatic diamines, such as m-phenylene diamine or bis(4-aminophenyl) ether.
They have outstanding thermic properties. Thus, films prepared from them are ideal materials for the preparation of flexible printed circuit boards.
As an example, a commercial polyimide film of "Kapton H" shows the following typical properties:
______________________________________ Density 1.4 g/cm.sup.3 Thickness 0.0075 . . . 0.125 mm Breaking-down voltage 140 . . . 270 kV/mm Tensile strength, lengthwise 170 N/mm.sup.2 Tensile strength, crosswise 170 N/mm.sup.2 Limiting temperature &gt;180.degree. C. ______________________________________
However, it was disadvantageous that so far the adhesives which were available for a bonding to metals, particularly to copper, were not heat-resistant enough, and did result in a to poor peeling resistance of the bonding.